The present invention relates generally to methods and systems for the precise calibration of instruments. More specifically, the present invention pertains to an accurate and efficient process for calibrating accelerometers.
An accelerometer is a transducer used for measuring acceleration. Acceleration is usually measured along a sensitive axis of the accelerometer. Generally, the magnitude of an applied acceleration is communicatively coupled to external instruments or circuits as an electrical impulse (e.g., voltage) having an amplitude proportional to the magnitude of the applied acceleration. The electrical impulse comprises the measured acceleration and is processed by the external circuits as required for a variety of applications. One such application is, for example, an Inertial Measurement Unit (IMU), where acceleration measurements are used to generate velocity and positioning information.
The electrical impulse output of an accelerometer is proportional to the applied acceleration. The process of calibrating an accelerometer consists of computing a constant of proportionality, referred to as a scale factor of the accelerometer. The scale factor of an accelerometer precisely relates the amplitude of the electrical impulses comprising the measured acceleration to the magnitude of a corresponding applied acceleration along the sensitive axis of the accelerometer. For optimal precision of measurement, it is desirable to calibrate the accelerometer by precisely determining the accelerometer""s scale factor.
Prior art systems for calibrating accelerometers (e.g., measuring and defining the scale factor) relied on comparisons of the accelerometers to certain standard devices. Such prior art systems necessarily assume that the standard devices themselves are properly calibrated, often leading to the introduction of additional error into the calibration process. For example, one prior art system (see prior art U.S. Pat. No. 5,970,779) requires the use of precisely controlled swing arm motor systems to which the accelerometer being tested is mounted, along with an appropriate counter weight. The swing arm motor would be precisely controlled by a processor to impart a simple harmonic motion acceleration to the sensitive axis of the accelerometer, and vary this acceleration by varying the angular acceleration of the swing arm. The resulting output of the accelerometer would be examined with respect to the controlled varying of the swing arm motor, and the scale factor would be determined therefrom.
One problem with the above prior art approach is that it requires a precisely controllable motor for varying the angular velocity of the accelerometer. The motor needs to precisely apply a simple harmonic acceleration to the accelerometer by varying the angular velocity about an axis of rotation. As described above, this system requires the proper calibration of the standard devices themselves (e.g., the motor), which often leads to additional error in the calibration of the accelerometer.
A second, more important drawback of the above prior art approach is that it requires measuring the radius of rotation of the accelerometer. This distance can be very difficult to measure accurately, since the measurement point of the accelerometer is internal to the accelerometer. Any error in this measurement will manifest itself in through a flawed calibration.
Thus what is required is a solution that accurately measures and determines the scale factor of an accelerometer without introducing unnecessary sources of error. The required solution should be precise and avoid reliance on standard devices, which can introduce error into the calibration process. The required solution should not rely on any time varying control of a standard device to impart variable acceleration. The present invention provides a novel solution to the above requirements.
The present invention provides a solution that accurately measures and defines the scale factor of an accelerometer. The method and system of the present invention is precise and avoids reliance on standard devices, which can introduce error into the calibration process. The present invention does not rely on any time varying control of a standard device to impart variable acceleration. It also does not rely on measuring the distance of the accelerometer from the axis of rotation.
In one embodiment, the present invention is implemented as a rotating turntable mechanism for determining a scale factor of an accelerometer. The scale factor is used to precisely calibrate the output of the accelerometer, ie to convert the output (voltage) of the accelerometer into units of acceleration. The accelerometer to be calibrated is mounted on a turntable mechanism. The turntable is configured so that the axis of rotation of the turntable is tilted with respect to the local gravity vector. The turntable is then spun around the axis of rotation at a constant angular velocity. This causes the accelerometer to experience a time varying component of the local gravity vector. The output of the accelerometer is logged as the accelerometer experiences the time varying component of the local gravity vector (e.g., due to the tilt angle). The logged output of the accelerometer is compared to a predicted output of the accelerometer (e.g., a sine wave), wherein the predicted output is based on the tilt angle of the turntable and the angular velocity of the turntable. This comparison is done in the spectral domain. This separates out the bias drift of the accelerometer and prevents this bias drift from corrupting the scale factor measurements. In so doing, the turntable mechanism of the present invention accurately measures and determines the scale factor of the accelerometer without relying on any time varying control of a standard device (e.g., stepper motor, etc.) to impart variable acceleration to the accelerometer, and without relying on a precise measurement of the radius of rotation of the accelerometer.